Public domain data

These data have no specific confidentiality restrictions for users. However, users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgment.

The recommended acknowledgment is

"This study uses data from the data source/organisation/programme, provided by the British Oceanographic Data Centre and funded by the funding body."

Neil Brown Smart Conductivity, Temperature and Depth profiler

The Smart CTD was developed by WHOI, Neil Brown Instrument Systems and MIT and consists of an internally recording CTD which can be programmable to accommodate various sampling schemes. The data retrieval is done via connectors to a computer, where it is digitally logged.

This instrument was designed to be used with minimal technical expertise and to be replaced if problems arise, rather than being repaired at sea.

RV Scotia 6/68 Leg 2 CTD Data Documentation

Introduction

Documentation for CTD data collected on R.V. Scotia Cruise 6/88 leg 2 (17 th - 28 th June 1988) by the Department of Agriculture and Fisheries for Scotland, Aberdeen, UK under the direction of E. Henderson.

Instrumentation

The instrument used was a Neil Brown Instrument Systems SMART CTD which logged data into its internal memory. Conductivity, temperature and pressure only were logged. The instrument has two modes of operation: either raw or averaged data can be collected. The former produces data at a rate of 5 scans/second and the latter 1 scan every 0.96 sec. So far the instrument has always been used in the averaging mode. An EPROM in the instrument contains a routine which does the averaging, and also introduces the time lags required for matching the responses of the different sensors. The internal memory of the instrument is 40,000 scans of data.

Sampling

The CTD was lowered, at rates of between 30 and 40 metres/minute, (i.e. 1 scan approximately every metre) to within 5-10 m of the sea floor (this depth being taken from soundings). Although CTD data were collected both on the downcast and the upcast; only the data from the downcast were used and compared with the bottle samples. The CTD data were then read back into the shipborne computer, an HP85, salinity was calculated and data plotted out - as a quick look check on the data.

A calibration station (using reversing thermometers and water bottles) was taken every 4-5 CTD stations. CTDs and bottle samples may be taken at the same site; in which case the CTD cast was carried out first, and the bottle station immediately afterwards - this only occurred when the bottle stations were required for chemical data. The ship's winches are too close together to allow the two to be done simultaneously. At the hydrographic stations 2, 3 or 4 bottle samples were taken together with reversing thermometer measurements.

Calibration

The CTD is returned occasionally to the manufacturer in the USA for re-calibration at the factory - this was done in January 1985 and again in April 1988. At the factory the re-calibration is burnt into the EPROM. Memory is available on the EPROM for the addition of software for bench testing temperature and pressure, if required. However the CTD is not calibrated at DAFS as there is no calibration facility. Although there is little drift in the instrument, over a couple of years there is sufficient to warrant returning the instrument to the factory.

Data Processing

In the laboratory, the data were transferred onto a VAX 11/750 computer together with the header information from the log. Any spikes encountered were treated in one of the following ways:

If the spike is small, (i.e. less than 4 observations) the data values were deleted, leaving a gap.

Data were interpolated for spikes of greater than 4 observations. Usually, for data below the thermocline a linear interpolation was applied; for data in the thermocline a cubic spline was employed.

Pressure, temperature and conductivity data values were supplied to BODC. Conductivity was converted to conductivity ratio and then converted to salinity using UNESCO recommended routines.

Caution

The following 2 series are very noisy over the depth range specified and should be treated with caution:

Originator's Reference

BODC Series Reference

Depth range (decibars)

27/06/0150

175408

0 - 19

27/06/0156

175470

9 - 27

Note also, the temperature, conductivity and salinity data from the top 12 decibars of the water column for Originator's Reference 27/06/0236 (i.e. BODC Series Reference 176344) appear suspect and should be treated with caution.